An electrochemical cell, e.g. a battery, generally includes a positive electrode (cathode during discharge), a negative electrode (anode during discharge), and an electrolyte containing one or more ionic species that act as charge carriers providing ion transport between the cathode and anode. During charge and discharge of the electrochemical cell, electrodes exchange ions with the electrolyte and exchange electrons with an external circuit (e.g. a load or a charger). Many widely available battery systems are based on cation electrode reactions, with electrodes capturing or releasing a cation from an electrolyte and balancing the charge with an electron from the external circuit. Because of its very low electrochemical oxidation/reduction potential and light weight, the element lithium (Li) is commonly used in cation based battery systems. Both lithium and Li-ion batteries are commercially available and widely used.
However, the electrochemistry of lithium metal or lithium-containing electrodes presents problems for commercial use. In this respect, lithium metal is highly reactive, and precautions must be used to in order to store lithium in safe forms (e.g., intercalates), thus increasing battery weight and reducing energy density. For example, individual Li-ion batteries and Li-ion battery packs often contain expensive voltage and thermal control circuitry to shut down the battery when voltage or temperature are outside an optimal operating range.
Fluoride-anion based electrode reactions offer an alternative to lithium and lithium-ion batteries. For example, in a fluoride ion battery (FIB), an anode and cathode are physically separated from one another but in common contact with a fluoride anion conducting electrolyte. The anode and cathode are typically formed from low potential elements or compounds (e.g., metals, metal fluorides, or intercalating compositions such as graphite), where the cathode material possesses a higher potential than the anode material. Fluoride anions (F−) in the fluoride anion conducting electrolyte move from the cathode to the anode during discharge, and from the anode to the cathode during charging of the battery.
Fluoride ion batteries potentially have high theoretical capacity due to their reaction mechanisms, in which both electrodes (anode and cathode) may participate in electron or ion transfer with the electrolyte.
So far, although the ideal cathode reaction, such as Cu+2F−↔CuF2+2e− for example, has been observed, the ideal anode reaction, such as CaF2+2e−↔Ca+2F− for example, has not been confirmed in non-aqueous organic electrolyte solutions because of the large overpotential of such reaction.